An optical time domain reflectometry (OTDR) system can be used to measure values of a physical parameter of interest along an optical fiber. In one application, the optical fiber can be deployed in a wellbore that is used to produce fluids from a reservoir in a subterranean structure, where the reservoir can include hydrocarbons, fresh water, or other fluids. Examples of physical parameters that may be of interest include temperature, strain, and other parameters. One type of OTDR system is the Brillouin OTDR system, which takes measurements based on Brillouin scattering.
Brillouin scattering is an inelastic phenomenon that results from the interaction of incident optical photons (of an incident optical signal) with acoustic phonons in the medium (the optical fiber) (Brillouin scattering occurs in media other than optical fibers). This interaction induces a counter-propagating optical wave (reflected or backscattered optical signal) having a frequency (Brillouin frequency) that is shifted from the frequency of the original incident optical wave. Brillouin scattering in an optical fiber is sensitive to both temperature and strain changes in the optical fiber.
Conventionally, Brillouin OTDR acquisition systems are designed to be used with single-mode optical fibers. The reason for the preference of single-mode optical fibers in conventional Brillouin OTDR systems stems from the relatively close frequency spacing between the Brillouin backscattered light and the probe pulse that is transmitted into the optical fiber, and the techniques used for separating probe and Brillouin signals are generally specific to, or more effective in, single-mode fibers than multimode fibres. However, requiring that Brillouin OTDR acquisitions be made with OTDR systems having single-mode optical fibers results in reduced flexibility of such OTDR systems.